The invention relates to a charge-coupled device comprising at a surface of a semiconductor body a system of juxtaposed parallel channels and a series output register of which successive charge storage and transfer elements are each associated with a parallel channel, which system of parallel channels comprises two sub-groups so arranged that the parallel channels belong alternately to a first and to a second sub-group, and in which there is provided at the area of transition from the parallel channels to the series output resistor an electrode system by means of which a row of charge packets transported through the parallel channels can be divided into two sub-rows which correspond to the two sub-groups and which can be introduced successively into the series output register, said electrode system comprising first and second comb-shaped electrodes and first and second strip-shaped control electrodes, the first comb-shaped electrode having a cross-bar in the form of a strip extending transversely across the parallel channels and having teeth which extend from the cross-bar in the direction of charge transport above the parallel channels of the first sub-group, the second comb-shaped electrode having a cross-bar which near the tips of the teeth of the first comb-shaped electrode extends transversely across the parallel channels and has teeth which are interdigitated with the teeth of the first comb-shaped electrode and extend above the parallel channels of the second sub-group into the proximity of the strip-shaped cross-bar of the first comb-shaped electrode, the first and second strip-shaped control electrodes extending transversely across the parallel channels and, viewed perpendicular to the surface, being present in the regions between the tips of the first comb-shaped electrode and the cross-bar of the second comb-shaped electrode, and in the regions between the tips of the second comb-shaped electrode and the cross-bar of the first comb-shaped electrode, respectively.
A known type of charge-coupled device having a system of parallel channels, the outputs of which are coupled to the parallel inputs of a series output register, is a series-parallel-series (SPS) memory. The parallel channels form a memory matrix for analog or digital information which is introduced via a series input channel and which can be read out via the series output channel. Another form of charge-coupled device of the above-described type is an image sensor in which the charge stored in the parallel section corresponds to a received two-dimensional radiation pattern. Although the invention can be used for such other forms of device and not merely in SPS-memories, it will nevertheless be described mainly with reference to SPS-memories due to the particular advantages for these important memory devices.
In conventional SPS-memories, the series channels are formed by 2-phase CCD's. Because in a 2-phase CCD one empty site must always occur per full charge storage site, it is apparent to choose the pitch between the parallel channels such that one parallel channel occurs in the series register per two charge storage transfer sites. Upon transferring a row of charge packets from the parallel section into the series channel, half of the storage sites in the series channels are occupied, so that the charge packets can be transported to the output in the usual manner.
A method of increasing the information density, known per se, for example, from U.S. Pat. No. 3,967,254, uses the principle of "interlacing" and "de-interlacing". The pitch between the parallel channels in comparison with the above-described construction may be chosen to be two times smaller so that one parallel channel occurs per charge storage/transfer site of the series input register and/or the series output register. The information density or quantity of information can thus be substantially doubled. Because only half of the site of the series channels can be occupied simultaneously, the information can no longer be read in or read out per column. Therefore, upon reading in, for example, first the even sites of a row are occupied with information and then, in a second step, the odd sites (interlacing). Analogously, when reading out a row, first the charge packets, for example, on the even sites are introduced and read out in the series output channel and then the information in the odd sites (de-interlacing) are introduced and read out.
The electrode system configuration at the parallel-to-series transition which as specified in the above comprises two interdigitated combs is known inter alia from the already-mentioned U.S. Pat. No. 3,967,254, and serves for de-interlacing the stored information. Its operation thereof is basically as follows: first a complete row of signal charges is moved below the said first comb-shaped electrode. The signals then alternate below a tooth and below a region below the cross-bar of the comb-shaped electrode. By means of the said first control electrode, the signals which are stored below the teeth of the first comb-shaped electrode can be moved via the regions below the cross-bar below the second comb-shaped electrode into the series-output register channel so as to be read out at the output. During this parallel-series transfer the signals, which are stored below the cross-bar of the first comb-shaped electrode, are not transferred since the first control electrode overlaps only the teeth and not the cross-bar of the first comb-shaped electrode. When the series output channel is again empty, the remaining signals can be moved below the teeth of the second comb-shaped electrode by means of the second control electrode and then again into the series-output channel.
In the last-mentioned transport, in which the charge is moved from below the teeth into the series-output channel, the electrode structure may give rise to problems. In case of small quantities of charge the charge transfer consists substantially of thermal diffusion in which the charge (on the source side) moves asymptotically to zero along an exponential curve as a function of the time t with a time constant .tau.=4L.sup.2 .multidot.(.pi..sup.2 D).sup.-1. In this formula L is the length of the electrode on the source side and D is the diffusion constant. Because the length L of the teeth generally is large, the charge transport will be rather inert. In a specific embodiment in which L at the area of the teeth is more than twice as large as the cross-bars of the comb-shaped electrodes (and, the lengths L of the remaining clock electrodes), the time constant T becomes more than 4 times as large.
The comb-configuration of the de-interlacing electrodes therefore has a detrimental influence on the frequency properties of the memory, in particular in those cases in which the further dimensions are chosen to be as small as possible.